In various industries, including the photoprocessing industry, aqueous solutions are susceptible to biogrowth under certain conditions. Biogrowth, that is growth of bacteria and/or fungus, can be a problem for many reasons of health, safety and quality in the food, chemical, photographic and health industries.
Attempts to reduce or eliminate the problem of biogrowth have included discharging solutions to the environment followed by cleaning of machines and processing tanks and equipment, adding chemical biocides, radiation treatment of solutions, and various filtration techniques. Because of biogrowth in the photographic industry, the useful life of photoprocessing solutions has been unnecessarily reduced, photoprocessing equipment has been cleaned too frequently, and fresh solutions are needed at additional labor and chemical costs.
Each of the known methods for reducing biogrowth has inherent disadvantages. For example, the use of chemical biocides gives uncertain results and requires the handling of expensive chemicals that are often hazardous. Frequently discharging solutions to the environment has obvious problems and is increasingly regulated in many countries. Ultraviolet radiation is not efficient for treating colored solutions. The addition of heavy metals such as silver and copper to control biogrowth is often in conflict with governmental regulations for discharge of effluent. The addition of silver to photographic processing solutions causes the formation of silver complexes that fail to inhibit microbial growth. Filters can be clogged quickly and require frequent replacement or cleaning, and can become microbial breeding grounds.
The use of heat to control biogrowth is well known, particularly in the food industry. Of the methods used to provide heat (including boiling and autoclaving), pasteurization involves exposure to high temperatures for a relatively brief time. In bulk pasteurization, the process typically involves heating a fluid to about 65.degree. C. for 30 minutes. In flash pasteurization, the fluid is heated to a higher temperature (over 70.degree. C.) for much less time (for example, 15 seconds).
Thermoelectric refrigeration is a known technology [CRC Handbook of Thermoelectrics, Rowe (Ed.), Inc. 1995, pp. 597-676]. This technique involves passing a current through one or more pairs of n- and p-type semiconductor materials, providing both hot and cold sides that can be contacted with fluids to be treated. At the cold side, direct current passes from the n- to the p-type semiconductor material and heat is absorbed from the environment (in this case, the treated fluid). The adsorbed heat is transferred through the semiconductor materials by electron transport to the other side of the junction and liberated as electrons return to a lower energy level in the p-type material. This phenomenon is called the Peltier effect, and devices used in this manner are often called Peltier heat transfer devices. More than one pair of semiconductors are usually assembled together to form single or multistage thermoelectric modules.
In the operation of such modules as heat transfer devices, usually one fluid is heated while another is cooled (see for example, see U.S. Pat. No. 3,506,543 of Hayes et al and U.S. Pat. No. 5,027,145 of Samuels). Alternatively, such heat transfer devices can be used to provide heat for evaporation of one fluid and condensation of another, as described for example in U.S. Pat. No. 4,316,774 (Trusch).
In the noted Samuels patent, a Peltier heat device is used in conjunction with photographic film processors that utilize both a developer bath and a water wash bath. During use of the heat device, the developer is cooled while the water wash is heated.
There is a need to provide an improved means for inhibiting biogrowth in various aqueous fluids, especially photographic processing solutions, in which the same fluid is treated on both sides of the Peltier heat transfer device to provide adding efficiencies.